From German Offenlegungsschrift No. (laid-open application) 25 54 988, which corresponds to U.S. patent application, Ser. No. 747,676, filed Dec. 6, 1976 and commonly assigned with the present application it is known to detect post combustion reaction exhaust gases leaving the combustion chamber with the aid of an ion current sensor disposed directly downstream of the outlet valves, and to control the air number .lambda. of the operating mixture delivered for combustion to the engine in a closed-loop fashion in accordance with the output signal of the ion current sensor when compared with an appropriate set-point value.
This closed-loop control method is used in an internal combustion engine having externally supplied ignition. In essence, it is the sharp increase in the ion current at the lean running limit of the engine which is evaluated. There, because of the very large air excess, the hydrocarbon content of the exhaust gas increases greatly, and because of the poor ignitability of the mixture, delayed combustion occurs more frequently. This is generally the cause of the increase of the ion current at the lean running limit. A further increase in the ion current, although less well-defined, is observed according to the publication referred to above in ranges for .lambda. of less than 1. However, the post combustion reaction range is limited by the free oxygen content in the exhaust gas, since the mixture itself is less than stoichiometric, so even this increase is not well-defined.
It is also known that self-igniting engines, upon approaching the air number 1, have a sharply increasing soot emission in the exhaust gas. Accordingly, in order to stay within the prescribed emission limits, it is necessary, in self-igniting engines, to maintain a sufficient safety margin for this increase in soot emission.
Furthermore, it is known to operate self-igniting engines in the air number ranges associated with the running limit in externally ignited engines, i.e., with large excesses of air, without producing interruptions in operation. This can be used for closed-loop control purposes. The sufficiently large safety margin from the air number .lambda. associated with this operation, which must be maintained to prevent smoke in the exhaust, is, however, disadvantageous. Because the output produced by the engine increases as the air number .lambda.=1 is approached, the result is a reduced exploitation of the output capacity of the engine.